1. Field of the Invention
The present invention relates to a manufacturing procedure for printed circuit boards. It is directed to those printed circuit boards which utilize one or more power devices.
2. Information Disclosure Statement
"Power devices" as used herein means semiconductors or resistors which dissipate significant amounts of heat and require assembly to a heat sink. These power devices have at least one mounting hole for attachment to a heat sink and two or more electrical leads for connection to a printed circuit board. When the heat sink is small in size, it is typically affixed to the printed circuit board with wave-solderable mechanical support pins. However, high-power devices require a heat sink that is usually too large to fit on a printed circuit board; said heat sink is usually a chassis-mounted component or is part of the chassis itself. In these instances, the printed circuit board must be so configured as to allow its attendant power devices to thermally interface (either directly, or, indirectly with a conducting device such as a heat pipe) with the off-board heat sink. During assembly, steps must be taken to assure that the electrical leads of the power devices align with the printed circuit board, while at the same time the mounting holes of the power devices align with the heat sink. Typically, the power devices are omitted when the printed circuit board is populated, wave-soldered, and washed. Subsequently, the power devices are loosely attached to the heat sink by means of mounting screws. This is followed by hand-fitting and hand-soldering the printed circuit board to the electrical leads of the power devices. Once the proper fit is established among the printed circuit board, the power devices, and the heat sink, the mounting screws are tightened.
The problem with the above-described assembly method is that automation cannot be used to solder and wash the electrical connections of the power devices, since the invariably bulky heat sink cannot be supported as the printed circuit board passes through the wave-solder machine and the board-washing machine. Hand soldering produces relatively inconsistent results, costs more, takes longer, and introduces human error. Also, the increased handling of the power devices places them at risk of being damaged by electrostatic discharge. In addition to the aforesaid reliability issue, this introduces the need for static dissipative tools, materials, clothing, and training; each of which represents an additional manufacturing expense. Further, the hand-soldered connections must either be left dirty, or manually cleaned with a spray product that could damage the finish of nearby painted or powder-coated surfaces or contaminate other chassis components with the flux residue rinsed from the connections. Thus, the present invention is directed to overcoming these prior art shortcomings.